OFDM is a modulation format that is used in many of the latest wireless and telecommunications standards. One example of the use of OFDM is “5G” cellular. Next generation or “5G” telecommunications technology represents a giant leap forward in both requirements and resources over the current Long Term Evolution (LTE) telecommunications technology. Under the 3rd Generation Partnership Project (3GPP), the “New Radio Access Technology,” often called “NR” (new radio), is being developed as the underlying physical layer technology enabling 5G. See, e.g., Zaidi et al., Waveform and Numerology to Support 5G Services and Requirements, IEEE Communications Magazine (November 2016), pages 90-98; and 3GPP TSG RAN WG1 Mtg #86 Tdoc R1-168526 (Sep. 2, 2016), draft 3GPP TR 38.802, Study of New Radio (NR) Access Technology—Physical Layer Aspects, which are both incorporated herein by reference in their entirety.
In terms of resources, it is expected that 5G may have access to frequency bands from under 6 GHz (where the current LTE frequency bands are) up to 100 GHz. In terms of requirements, three 5G categories are often discussed:                enhanced mobile broadband (eMBB), requiring very high data rates and large bandwidths;        Ultra-reliable low latency communications (URLLC), requiring very low latency, and very high reliability and availability; and        Massive machine type communications (mMTC), requiring low bandwidth, high connectivity, enhanced coverage, and low energy consumption on the user end.        
One aspect of the 5G technologies is the changes to the physical layer, in which, as mentioned above, is often referred to as NR by the 3GPP. Numerology (i.e., subcarrier spacing (SCS) and waveform parameters, such as the cyclic prefix (CP)) is presently a non-issue because, in LTE, there is only one numerology in which, for example, the SCS is always 15 kHz. In a radio environment such as the current LTE environment, it is a relatively simple task for a user equipment (UE) to roughly synchronize with the signal and, based on their preset mapping in the frequency domain, find the primary synchronization signals (PSSs) and secondary synchronization signals (SSSs) in the time domain to fully synchronize.
By contrast, because of the range of 5G requirements, NR must have multiple numerologies in order to encompass the range of simultaneous usage (from relatively low bandwidth, like mMTC, to extremely high bandwidth, like 4K video on eMBB). In practice, this means, for example, there may be multiple SCSs, such as, for example, 15 kHz, 30 kHz, and 60 kHz, of different numerologies transmitting at the same time and on at least partially overlapping frequency bands.
Thus, a UE in 5G NR must be able to determine, isolate, and synchronize to more than one numerology—a new requirement for the UE.
Similar issues may arise in other communication systems where OFDM signal may be located near OFDM signals with different numerology, non-OFDM signals or OFDM signals that have the same numerology but have frequency offset of fraction of subcarrier or symbol position time offset.
All of the situations described above will give rise to intercarrier interference (ICI) into the OFDM signal.